A spot welding electrode must have high electrical conductivity, high thermal conductivity, and superior high temperature strength. Accordingly, various precipitation hardening type copper alloys, such as chromium copper, zirconium copper, and zirconium chromium copper, have been widely used. Recently, an electrode of alumina dispersion strengthened copper, which disperses fine grain alumina into the copper base, has been brought into practical use to significantly improve heat resistance and deposition durability.
The above described spot welding electrodes are applied to the spot welding of aluminum and aluminum alloys (referred to simply as "aluminum" hereafter). When these electrodes are applied to a recent highly automated, high speed progressive aluminum spot welding work, abrasion resistance at the tip of electrode and the strength of the spot-welded part often become problems.
To solve these problems, unexamined Japanese Patent publication 63-260684 provided an electrode forming a thin metallic film, such as W, having a high hardness and high melting point thereon at the area contacting with the material being welded. The electrode is, however, expensive and has insufficient workability, and still leaves the problem of practical application.
Unexamined Japanese Patent publication 63-30185 proposed electrodes coated by Co base alloy and Cr base alloy at the tip thereof, and examined Japanese Patent publication 60-48275 proposed an Al-coated electrode at the tip thereof. However, Co and Cr have problems in cost and electrode durability. As for the Al-coated electrode, sufficient preliminary treatment of aluminum members being welded to remove oxide film therefrom is necessary to assure prolonged electrode life and sufficient strength at the welded part. Such removal of oxide film takes time, and, in some cases, requires use of a wire-brush for perfect removal of the film.